Landfilling and landfill gas. Landfilling and dumping dominate waste disposal in the United States, as well as waste disposal worldwide. The U.S. Environmental Protection Agency (EPA) estimates U.S. landfilling of Municipal Solid Waste (“MSW”) at about 160 million tons annually over the past few years (U.S. EPA 2002) And, worldwide, even greater tonnages of organic solid wastes, several times those of the US, are landfilled and otherwise buried in dumps around the world
In wastes either buried in landfills or simply dumped, organic components decompose to form “landfill gas” (LFG) or biogas. LFG (biogas) comprises approximately equal volumes of methane and carbon dioxide, with lesser amounts of other gases and moderate levels of pollutants. A representative reaction for methane generation from cellulose, the largest fraction of most organic waste, is:
Importance of Landfill Gas Recovery.
The landfill gas (“LFG”) has pronounced environmental impacts, and its recovery is of extremely high importance across the United States (U.S.), and worldwide, for a variety of related reasons:    1. Mitigating climate or “greenhouse” effect The climate or “greenhouse” effect of methane emitted from landfills is a major global concern, simply because of enormous amounts of organic wastes, hence methane emissions, involved. In climate terms, methane from landfills adds to (makes a difference of) 3 to 10% in the annual increase in radiative forcing due to buildup of all greenhouse gases in earth's atmosphere. In more simplified terms, the presence or absence of landfill methane emissions into the atmosphere can be considered to make a difference of about 3 to 10% in the “greenhouse effect”    2. Potential for renewable LFG energy The landfill methane recovered from landfills can be important as a fuel. It is usable with existing technology and equipment (Augenstein and Pacey, 1992). Available U.S. landfill methane, potentially recoverable (as of now, based on the US Energy Information Agency (EIA) and other statistics) but not exploited for energy, conservatively equates to the energy value of over 150,000 barrels of oil a day.
This amount of energy is significant in terms of improved national security and energy self-sufficiency for the U.S. Furthermore, because methane from wastes is nearly all from photosynthetically fixed carbon, this methane is a renewable fuel. It displaces the use of fossil carbon fuels, thereby lessening climate effects of fossil CO2.    3. Other important reasons for landfill gas capture. Other important reasons for landfill gas control and recovery include (a) to mitigate methane effects on stratospheric ozone destruction, (b) to prevent emission of local air pollutants, (c) to mitigate of landfill methane migration and explosion hazards, and as well as odor problems, and (d) to develop practical, economic and cost-effective options for voluntary or non-voluntary greenhouse gas abatement actions, and practical options for carbon sequestration.
For all of these reasons, the recovery of landfill gas at high efficiency is a high priority to regulators as well as landfill owners and operators.
Notwithstanding the potential benefits of methane recovery, “conventional” landfill gas extraction is relatively inefficient.
Conventional Gas Extraction with Wells or Trenches.
The usual gas recovery approach is to use deep wells attached to a network of pipes and a gas pump (blower) that applies vacuum to extract the gas from waste. To illustrate performance of conventional systems, gas flow dynamics with “conventional” well (or trench) extraction are shown qualitatively in FIG. 1. FIG. 1 shows landfill 1 containing waste 2. A well 3 collects biogas from the landfill. Cover layers 4 are in contact with the atmosphere at the surface of the landfill. Arrows in FIG. 1 denote directions of gas fluxes, through (in and out of) a waste landfill surface, and within the waste. Gas flow velocity is denoted qualitatively by lengths of the arrows. Note the gas escaping to the atmosphere far from the wells. It is principally because of this LFG emission and loss far from the wells that gas capture is typically 60-85% (SWANA 1994. Solid Waste Association of North America Workshop on Landfill Gas Modeling and Recovery, 1994. Personal communications from participants). This inefficiency is acknowledged and estimated at 75% by the U.S. EPA (EPA, Peer et al. 1991, ICF, 2002) and California Air Resources Board. The inefficiency has been an accepted feature of extraction.
The profile of surface emission flux is recognized to lead to potential for some emissions away from the wells under most circumstances. Note also that there is almost always entrainment of gas, whether LFG or atmospheric air, through the surface area most proximate to deep collection. Both LFG emission far from wells, and air entrainment proximate to subsurface collection, are well recognized as deleterious to collection efficiency. A “tradeoff” exists between extracting or “pulling” at too high a flow rate and thereby entraining excessive atmospheric air, versus pulling too little and recovering less LFG. This poses one dilemma of conventional extraction.
Geomembrane over highly conductive layer: Zison. An invention that partially ameliorates inefficiency and air entrainment problems of gas collection by wells has been to collect by a surface geomembrane or a low permeability layer over a surface or near-surface highly conductive layer. (Zison, U.S. Pat. No. 4,442,901). A schematic of the Zison highly conductive layer recovery method is shown in FIG. 2. In FIG. 2, biogas is emitted by digestion of waste 2. The arrows show flux of the gas. Overlying the bulk of the waste is a gas-permeable layer 5. A surface geomembrane 6 is used to prevent gas escape from the gas permeable layer. Biogas is extracted (arrow 21) from the gas permeable layer 5.Adjustment (“monitoring” or “tuning”) of well extraction rates. A gas extraction system based on wells as in FIG. 1 cannot be simply installed and turned on. It must be adjusted (“tuned”) to maximize recovery. Typical tuning gradually increases extraction rates from wells over time, until falling extracted methane levels at the wellhead indicate that air entrainment through the landfill surface, and into the collected gas, is too high. If methane content falls too far, the extraction rate must be reduced. Not only are LFG emissions far from wells, and the air entrainment proximate to subsurface collection recognized as deleterious to collection efficiency, but a “tradeoff” necessarily exists between extracting or “pulling” at too high a flow rate and thereby entraining excessive atmospheric air, versus pulling too little and recovering less LFG. Although gas extraction at sufficiently high rate to reduce gas pressure in the landfilled waste to below atmospheric can eliminate nearly all fugitive LFG emissions, the associated air entrainment dilutes gas and creates problems with energy uses.The practice of constant rate extraction. Current conventional extraction practice (barring mishaps) has been to extract LFG at a constant rate 24 hours a day. For one thing, this is the easiest LFG extraction management approach, minimizing need and effort for operator adjustment. Once extraction rates, typically established by monitoring or “tuning,” are established, the well vacuum and corresponding extraction rate are left alone for weeks or months that elapse until the next needed adjustment is made. Constant-rate LFG recovery has developed as standard practice in the LFG industry because it has been believed—and is widely validated by accepted measurement techniques—that an extraction rate near or slightly exceeding generation rate controls surface gas flux (both air in and LFG out) and, thus, controls surface methane emissions at least moderately well, i.e. to extents acceptable to operators and regulators. For most landfills, there is as noted already some degree of “overpull”, i.e. extraction in excess of generation, so that rather significant air entrainment occurs, as indicated by gas compositions containing 5-30+% of atmospheric N2 gas. (O2 is absent or present in small amounts, being depleted by microbial consumption as the air passes through the landfill). This overpull has been found associated with best control.
New landfill designs that facilitate collection of landfill gas (biogas), and new methods of collecting biogas generated in landfills are needed. Preferably the methods and designs will allow for more efficient collection of biogas than previous methods (i.e., allowing less biogas to escape). Preferably the methods and designs also minimize collection of atmospheric air with the biogas and minimize the drawdown of air into landfills. Air contamination harms the quality and utility of collected biogas, and air drawn into a landfill creates a more oxidizing environment in the landfill that leads to consumption of methane by oxidation and inhibits the anaerobic microbial fermentation that produces biogas.
Preferably the LFG extraction methods would allow efficient and most advantageous use of the biogas collected. Many examples exist of cases where energy is more advantageous and valuable at certain times than others. A case in point is electricity demand and revenue from electricity sale. For most of the US the peak demand for electricity occurs during the day. The plants that provide for daytime or “peaking” electricity demand may run only during the day, that is, about half time. The power produced by such plants that run only about half time is correspondingly more expensive inasmuch as the capital and other fixed cost must be allocated over the plants' shorter run times. Thus the power necessary to meet daytime needs, or “peaking power” is more expensive than continuously generated: “baseload” power. If landfill gas can fuel power selectively during the daytime, that power can be sold at a premium that power necessary during the daytime can command. This is but one example of energy and power that is more valuable at some times than others. An Oct. 17, 2007, Wall Street Journal Article (“Electricity Demand Is Far Outpacing New-Supply Sources,” by Rebecca Smith) states that growing demands along with delays in new power plant construction are such that demand threatens to outpace supply across the US. The Wall Street Journal article notes that much the greatest shortage is likely in “peaking” plants that must operate when necessary to meet maximum demands.
From the above it is clear that it would be desirable to use landfill gas selectively at times when its energy is most desirable.
To fuel “peaking energy” needs (such as heat or electrical power) with landfill gas, it is necessary that the extracted landfill gas fuel (biogas) be preferentially deliverable to the point of use at times of maximum energy need. One way to do this would of course be by storage in a storage volume or reservoir external to the landfill such as a storage tank. However all means of such external storage turn out to add expense that is prohibitive relative to the value or premium that may be received for peaking energy.